xBtg-x regulates Wnt/β-Catenin signaling during early Xenopus development

In Xenopus, two signaling systems, maternal β-Catenin and Nodal-related, are required for induction of the Spemann organizer and establishment of the body plan. By screening cDNA macroarrays for genes activated by these two signaling pathways, we identified Xenopus xBtg-x, a novel member of the Btg/Tob gene family of antiproliferative proteins. We show that xBtg-x is expressed in the dorsal mesendoderm (Spemann organizer tissue) of gastrula stage embryos and that its expression is regulated by both β-Catenin and Nodal-related signals. Microinjection of synthetic xBtg-x mRNA into Xenopus embryos induced axis duplication and completely rescued the ventralizing effects of UV irradiation through the activation of the canonical Wnt/β-Catenin signaling pathway. Interestingly, xBtg-x stimulated β-Catenin-dependent transcription without affecting the stability of β-Catenin protein. These data suggest that xBtg-x is a novel component of the Wnt/β-Catenin signaling pathway regulating early embryonic patterning.